Inside a typical digestive tract are around 100 trillion active bacteria, yeasts, fungi, viruses and protozoans. This vast ecosystem of organisms is known as our gut microbiome and collectively weighs up to 2kg (heavier than the average human brain), with more than 1,000 bacterial species identified in an adult gut. These different types of bacteria can be put into three groups – beneficial, potentially pathogenic (opportunists) and pathogenic. However, all 3 groups of different types of bacteria is normal in a healthy gut – it’s getting the right balance that is important. The beneficial bacteria (such as Lactobacilli and Bifidobacteria) help to maintain good gut health and overall well-being.
These bacteria carry out many metabolic processes in the human body, including the breakdown of non-digestible dietary residue to generate short chain fatty acids1 and the synthesis of some vitamins (such as K and B)2,3.
There is much still to be learnt regarding the composition, function and role of the microbiome, particularly in relation to if and how dysbiosis (an imbalance in gut bacteria) affects health. However, there is increasing evidence that a good balance of gut bacteria has a positive impact on the gut itself, which is perhaps not surprising. However, the research on the microbiome is now going further, and looking at its effect on other body systems, as diverse as mental health, skin and even weight management.
The idea that the microbiome existed first emerged around 100 years ago, but it is only in the last 10 years that research and knowledge into it has dramatically increased. In 2007 the international Human Microbiome Project was launched to try to characterise the microbial communities found in the human body and identify the role each microorganism has on health.
What affects the gut microbiome?
Just as no two humans are the same there are no two microbial makeups that are the same – even in identical twins. It is believed colonisation of the intestines starts at birth, or even in the womb. This early bacterial makeup then changes over time with exposure to various things that can shape or upset the balance of bacteria. The most dramatic changes in composition occur in infancy and early childhood. It is known that babies delivered vaginally have variations in the type of bacteria that colonise their gut compared to those delivered via cesarean section5,6. The type of feed they receive (breastmilk or formula) can also have an effect, as well as maternal nutritional status (overweight or undernourished) and use of antibiotics in early years.
Throughout life, the composition of the gut microbiome can continue to change. Some researchers suggest that modern lifestyles deplete, reduce diversity of, or unbalance the microbiome. For example, antibiotics are designed to kill specific harmful bacteria, but they have also been shown to lead to a reduction in the diversity of the gut ecology7,8. Shifts in microbiome can happen very quickly and, for many people, just one course of antibiotics would be enough to do this.
In the same way that antibiotics may be harmful, antiseptic modern lifestyles, which can reduce exposure to good and bad bacteria, are thought to have a similar effect.
Stress, whether in childhood or adulthood, may also have an influence on the composition of the microbiota9,10,11 as could diet – with a diet rich in fruits, vegetables and fibres claimed to result in a higher richness and diversity of the gut microbiota7,12,13.
How can changes be made to the gut microbiome?
Dietary and lifestyle changes – such as reducing stress, improving sleep and increasing exercise – are all thought to have a positive effect on the microbiome. Many people now also introduce live beneficial bacteria into their gut through taking a daily supplement.
- Janssen AWF, Kersten S. The role of the gut microbiota in the metabolic health. The FASEB Journal. 2015;29(8):3111-23.
- Hill MJ. Intestinal flora and endogenous vitamin synthesis. European Journal of Cancer and Prevention. 1997;6:43 – 5.
- Tappenden KA, Deutsch AS. The Physiological Relevance of the Intestinal Microbiota – Contributions to Human Health. Journal of the American College of Nutrition. 2007;26(6):679S-83S.
- Round JL, Mazmanian SK. The gut microbiota shapes intestinal immune responses during health and disease. Nat Rev Immunol. 2009;9(5):313-23.
- Nicolson JK, Holmes E, Wilson ID. Gut microorganisms, mammalian metabolism and personalised health care. Nat Rev Microbiol. 2005;3(5):431-8.
- Fallani M, Young D, Scott J, Norin E, Amarri S, Adam R, et al. Intestinal microbiota of 6-week-old infants across Europe: geographic influence beyond delivery mode, breastfeeding and antibiotics. J Pediatr Gastroenterol Nutr. 2010;51(1):77-84.
- Jandhyala SM, Talukdar R, Subramanyam C, Vuyyuru H, Sasikala M, Nageshwar Reddy D. Role of the normal gut microbiota. World J Gastroenterol. 2015;21(29):8787-803.
- Modi SR, Collins JJ, Relamn DA. Antibiotics and the gut microbiota. J Clin Invest. 2014;124(10):4212-8.
- Cryan JF. Stress and the microbiota-gut-brain axis: An evolving concept in psychiatry. Can J Psychiatry. 2016;61(4):201-3.
- De Palma G, Collins SM, Bercik P, Verdu EF. The microbiota-gut-brain axis in gastrointestinal disorders: stressed bugs, stressed brain or both? J Physiol 2014;592(14):2989-97.
- Dinan TG, Stanton C, Cryan JF. Psychobiotics: a novel class of psychotropic. Biol Psychiatry. 2013;74(10):720-6.
- Westerbeek EAM, van den Berg A, Lafeber HN, Knol J, Fetter WPF, van Elburg RM. The intestinal bacterial colonisation in preterm infants: A review of the literature. Clinical Nutrition. 2006;25(3):361-8.
- Scott KP, Gratz SW, Sheridan PO, Flint HJ, Duncan SH. The influence of diet on the gut microbiota. Pharmacol Res. 2013;69(1):52-60.